This invention pertains generally to the field of electrolytic capacitors (xe2x80x9celcapsxe2x80x9d) and specifically to covers and the construction of covers for capacitors. It is contemplated that the invention will be particularly applicable to aluminum electrolytic capacitors.
Electrolytic capacitors, and specifically aluminum electrolytic capacitors, generate internal heat during operation because of fluctuating. current (xe2x80x9cripple currentxe2x80x9d) and internal resistance (Effective Series Resistancexe2x80x94xe2x80x9cESRxe2x80x9d) in accordance with the formula: Power (watts)=I2rsxc3x97ESR. The heat is generated internally in the active element of the roll and must diffuse outward to the packaging (or can) before it can be carried away by convection, conduction and/or radiation to the ambient environment. Radial and axial heat flows serve to conduct the heat from the core of the capacitor to the sides and bottom of a cylindrical package in which the capacitor may be encased. Construction details of the capacitor can facilitate or introduce resistance to these heat flows. Excessive internal heat can increase the electrolyte temperature and exacerbate corrosion problems.
The tabs and terminals of elcaps are at special risk of corroding since they are not in contact with the normal working electrolyte, but rather are in contact with the vapors and condensate of that electrolyte. The vapor or condensate is normally less corrosion-resistant because of higher levels of halide (especially chloride) contamination than the normal working electrolyte, preferential volatilization of solvent and less desirable electrolyte components. One likely source of chloride (and other contaminants) is the molded cover (or header or lid) through which electrical connection is made. If the electrolyte vapors and condensate extract any appreciable amounts of halides from the cover, it is mostly likely to migrate under the electric field to the terminals and tabs causing corrosion.
The cover material for aluminum electrolytic capacitors has traditionally been molded from either Nylon(copyright) or standard phenolic molding compositions, which contain porous fillers. These materials are satisfactory for use with older, and more corrosion inhibiting, electrolytes at low temperatures (xcx9c85xc2x0 C.), and for lifetimes up to 2,000 hours.
For electrolytic capacitors that are intended to operate at temperatures of greater than 85xc2x0 C., alternate solvents such as dimethylformamide (DMF) or butyrolactone can be chosen, since they have less tendency to corrode or react with the electrolytic capacitor components at these temperatures (as do the various glycol-based formulations that are commonly used for lower temperature applications). These alternate solvents, however, have undesirable properties such as carcinogenicity, teratogenicity, toxicity, high costs and a tendency to damage the cover, gaskets, etc. It is desirable to use ethylene glycol solvents and solve the corrosive tendencies associated with them for higher temperatures and longer times.
The development of longer life, higher temperature elcaps using environmentally acceptable ethylene glycol has been blocked, at least partly, by the lack of clean, durable elcap covers that are commercially available and inexpensive to manufacture as well as crack and craze. Polymeric molding compositions used in the commercial manufacture of elcap covers comprise thermoplastic or thermosetting resins (such as phenolic resins), fillers and other additives (such as pigments, lubricants and processing aids). Thermoplastics such as Nylon(copyright) and polypropylene are flammable and can soften at high temperatures as well as crack and craze. Thermoset materials (such as phenolic resins) are mechanically more stable for long term, high temperature applications. Phenolic resins are typically reaction products of phenols and aldehydes, such as formaldehyde. Fillers can be used up to about 50 volume percent of the molding compositions, and include wood flour, paper, cotton flock, minerals, chopped cloth, fibrous glass, etc. Such fillers can be classified as porous or nonporous.
Phenolic molding compositions can be divided into six general groups, although there is some overlapping. Briefly, these include (1) general-purpose (usually wood flour-filled); (2) impact (filled with cellulose, minerals, glass fibers); (3) nonbleeding (wood flour-filled with carbon black pigment); (4) electrical (mineral-filled with very low water absorption and improved insulation resistance); (5) heat resistant (normally mineral-filled); and (6) special (compounds developed for specific chemical resistance, etc. which may have unique combinations of properties). See xe2x80x9cPhenolics,xe2x80x9d Modern Plastics Encyclopedia, Vol. 60, pp. 341-35. (McGraw-Hill, 1983).
Unfortunately, the standard phenolic molding compositions used for the manufacture of elcap covers permit the extraction of corrosion causing halides (principally chlorides) from the inner surfaces of the cover. This halide extraction characteristic of standard covers, especially in combination with the low corrosion inhibition capability of high performance electrolytes, increases the susceptibility of the elcap to corrosion and precludes the construction of reliable, long life elcaps which can be used at higher temperatures.
Other commercially available cover materials, such as Nylon(copyright), prevent to a greater extent the extraction of halides but are flammable, mechanically weak and subject to melting, cracking, crazing and other forms of deterioration. Some expensive specialty polymers exist which may be suitable for the manufacture of elcap covers, but they are not practical for commercial use due to their low availability and/or high cost
Accordingly, a clear need exists for improved molding compositions useful for the manufacture of elcap covers that are economical to produce, that can withstand high temperatures (even when used with high performance electrolytes with their lower corrosion inhibiting properties), and that offer long operating life and mechanical strength.
It is an object of the present application to provide a process for producing elcap covers that inhibits or eliminates the extraction of halides, particularly chlorides, from the cover during operation and which process can use commercially available molding compositions. It is also an object of the present invention to provide improved elcap covers which exhibit lower extraction of halides during operation. Further, it is an object of the present invention to provide elcap covers that have low halides extractability characteristics while maintaining suitable mechanical strength, low flammability and long service life. Finally, it is also an object of the invention to provide covers which are suitable for use in electrolytic capacitors at higher temperature and with high performance electrolytes.
These and other objects and advantages will become apparent to those skilled in the art upon a reading of the present disclosure.
The present invention provides manufacturing methods for the production of electrolytic capacitor covers (from improved molding compositions) that are economical, durable, and exhibit low extractability of corrosion causing halides, while preserving suitable mechanical and electrical properties. The invention also provides improved elcap covers manufactured in accordance with the disclosed methods and with the disclosed molding compositions. The manufacturing technique provided by the present invention requires careful selection of molding resins and nonporous fillers, minimizing porous fillers, in the formulation of the molding compositions for the elcap covers.
The invention will be better understood by reference to the following description taken in conjunction with the appended claims.
This section details the preferred embodiments of the subject invention. These embodiments are set forth to illustrate the invention, but are not to be construed as limiting. Since the present disclosure is not a primer on the manufacture of electrolytic capacitor covers, but rather relates to improved construction materials, manufacturing methods, and their selection, basic concepts and standard capacitor features known to those skilled in the art are not set forth in detail. Details for concepts such as choosing appropriate materials, solvents or operating temperatures or manufacturing pressures etc. are known or readily determinable by those skilled in the art. Attention is directed to the appropriate texts and references known to those skilled in the art for details regarding these and other concepts which may be required in the practice of the invention; see, for example, Deeley, Electrolvtic Capacitors (Cornell-Dubilier Electric Corp., So. Plainfield, N.J.xe2x80x941938), the disclosure of which is hereby incorporated by reference into the present disclosure to aid in the practice of the invention
Covers are generally molded using a suitable polymer resin/binder system in combination with one or more fillers. Usually, manufacturers use porous fillers because of the resulting increased moldability. Wood flour or fiber is a typical example of a porous filler. Injection molding is widely used in the industry for molding both thermoset and thermoplastic resins because it is the simplest, least labor-intensive process. However, conventional phenolic molding compositions may require substantial proportions of porous fillers such as wood flour to be moldable enough for employment with this method For compositions which are less moldable, one may resort to transfer or compression molding methods, although they are generally more labor-intensive.
Unexpectedly, it has been found that moldable, mechanically strong covers could be manufactured from molding materials that do not contain substantial amounts of porous fillers. While not wishing to be bound by any theory, it appears that the type of filler used in the molding composition may affect the porosity or permeability of the molded covers, and thus the tendency of the capacitor electrolyte to extract halides from the material. For example, the wood fiber fillers conventionally used in such phenolic molding compositions are relatively porous, thus allowing the electrolyte to infiltrate the molded product and extract halides from available surfaces of the molded material.
In accordance with the present invention, it was found that combining a thermoset resin, such as a phenolic molding resin having low halide extractibility, with a suitable nonporous filler can produce elcap covers having satisfactory properties. While phenolic resins are presently preferred, any thermoset resin which remains moldable when combined with nonporous fillers, e.g. mineral fillers, can be used. For example, amino resins such as melamine and urea resins, thermoset polyimides and polyphenylene sulfides can be used. The molding resin also preferably has low bulk values such as extractable halide and sulfate. Phenolic resins are presently preferred because of their moderate cost, compatibility with glycol and high structural durability. Whatever resin is used, the molding composition (and thus the molded covers) may consist essentially of the resin, although this is not preferred. Since improved physical properties and economy are obtained, molding compositions preferably contain fillers, which can be present up to 90 weight percent. The filled compositions can contain preferably from about 25 to about 75 weight percent resin, and most preferably about from 30 to about 60 weight percent.
Conventional organic, porous fillers such as wood flour are preferably absent, and are replaced by suitable nonporous or mineral fillers which are preferably low in extractable halides to reduce the extractability of halides from the moldings. Small amounts (up to 5 weight percent) of porous fillers may be present, provided the standard for low halide extractability is met. The inventive compositions can contain up to about 90 weight percent of a nonporous filler, including mineral fiber, and preferably contain from about 25 to about 75, and more preferably from about 30 to about 60 weight percent.
Suitable nonporous fillers are typically minerals, including mica, alumina, silica, calcium carbonate, various clays, talc and feldspar. Manufactured materials including glass fibers, glass spheres, solid ceramic spheres formed from fly ash, hollow spheres of sodium silicate or alumina, aramid fibers and hybrid fibers containing aramid and/or carbon can be used as nonporous fillers. Preferably the filler includes a small but effective proportion of a mineral fiber such as wallastonite, wallastakup glass fibers or asbestos, as this improves the physical properties of the moldings. The mineral fibers can constitute 2 to 20 or from about 1 to about 10, preferably from about 2 to about 8, and most preferably from about 3 to about 6 weight percent of the molding composition. The molding compositions can also contain small but effective amounts of pigments, lubricants, processing aids and similar additives known in the art.
Elcap covers manufactured in accordance with the present invention have xe2x80x9clow halide extractability,xe2x80x9d meaning that the molded covers yield less than 1 part per million extracted halide when tested. The moldings are tested by immersion in water at 95xc2x0 C. for 64 hours, followed by testing of the water by ion chromatography or similar method for the quantity of halide extracted. The extracted halide should be less than 1 ppm, based upon the molding weight. The halide extractability of the molded material is affected by both the halide content and porosity of the filler(s) and the nature of the polymerized resin. The xe2x80x9cextraction ratio,xe2x80x9d the ratio of extracted halide to bulk halide in the molding, indicates the degree to which halides may be extracted from a molded composition under such test conditions. Preferably, the molded covers-have extraction ratios less than about 0.1.
A particularly suitable molding composition is available commercially as Plenco(copyright) 06415, a molding powder available from Plenco, Inc., Sheboygan, Wis., which exhibits low halide extractability because of the use of mineral fillers (which are low in halides) and the reduction of porosity in the fillers. The filler is substantially nonporous, i.e. relatively impervious to electrolytes used in electrolytic capacitors, and low in halides, yet retains the property of moldability in the compositions to facilitate the manufacture of the elcap covers. Preferred filler materials for these compositions include blends of minerals with small but effective amounts of mineral fibers, because they have been successfully employed and tested and appear to improve the physical properties of the moldings.
The molded covers of the invention feature low halide extractability, by which it is meant that when immersed in water at 95xc2x0 C. for 64 hours, less than 1 part per million of halide ion (based upon the molding weight) is extracted The extracted halide can be measured by ion chromatography or any other suitable analytical method.